1. Field of the Invention
The present invention relates to a thin-film piezoelectric resonator, a filter and a voltage-controlled oscillator. Particularly, the invention relates to a thin-film piezoelectric resonator using longitudinal vibration in a direction of thickness of a piezoelectric thin film, a filter provided with the thin-film piezoelectric resonator, and a voltage-controlled oscillator provided with the filter.
2. Description of the Related Art
A surface acoustic wave (hereinafter referred to as “SAW” simply) device is used in a radio frequency (RF) filter or an intermediate frequency (IF) filter for forming a mobile communication apparatus or a voltage-controlled oscillator (hereinafter referred to as “VCO” simply) included in the mobile communication apparatus. Because the resonance frequency of the SAW device is inversely proportional to the distance between comb-shaped electrodes, the distance between comb-shaped electrodes is not longer than 1 μm in a frequency region exceeding 1 GHz. For this reason, the SAW device is apt to be hard to use when the frequency to be used becomes high.
As a substitute for the SAW device, a thin-film piezoelectric resonator using a longitudinal vibration mode in a direction of the film thickness of a thin-film piezoelectric material has attracted public attention in recent years. The thin-film piezoelectric resonator is called FBRA (Film Bulk Acoustic Resonator) or BAW (Bulk Acoustic Resonator). In the thin-film piezoelectric resonator, the resonance frequency is decided on the basis of the acoustic velocity and film thickness of the thin-film piezoelectric material. Generally, a resonance frequency of 2 GHz is obtained when the film thickness of the thin-film piezoelectric resonator is in a range of from 1 μm to 2 μm. A resonance frequency of 5 GHz is obtained when the film thickness of the thin-film piezoelectric resonator is in a range of from 0.4 μm to 0.8 μm. Accordingly, increase in frequency up to tens of GHz can be achieved in the recent film-forming technique.
A technique in which a ladder-type filter 102 formed of thin-film piezoelectric resonators 101 as shown in FIG. 39 is used as an RF filter of a mobile communication apparatus has been disclosed in IEEE TRANSECTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 43, NO. 12, p. 2933, DECEMBER 1995. The ladder-type filter 102 is formed of thin-film piezoelectric resonators 101 connected in series and in parallel between an input terminal Pin and an output terminal Pout. As shown in FIG. 40, a thin-film piezoelectric resonator 101 can be combined with a variable capacitor 104 and an amplifier 105 to form a VCO 103 of a mobile communication apparatus.
A structure of a currently most typical thin-film piezoelectric resonator and a method for producing the thin-film piezoelectric resonator have been disclosed in JP 2000-69594 A. The method for producing the thin-film piezoelectric resonator is as follows. First, a hollow is formed in a surface of a silicon (Si) substrate by anisotropic etching. A sacrificial layer is then formed on the substrate. For example, a boron phosphorus-doped silicate glass (BPSG) layer is used as the sacrificial layer. Then, a surface of the sacrificial layer is polished till the surface of the Si substrate is exposed, so that the surface of the sacrificial layer is flattened. As a result, while the sacrificial layer is embedded in the hollow preliminarily formed in the Si substrate, the surface of the Si substrate around the sacrificial layer can be exposed. A lower electrode, a piezoelectric material and an upper electrode are further laminated successively on the sacrificial layer. Then, a hole is formed till the sacrificial layer. The sacrificial layer is removed by selective etching through the hole, so that a cavity corresponding to the preliminarily formed hollow is formed between the Si substrate and the lower electrode. When these series of production steps is completed, a thin-film piezoelectric resonator is finished.
Proc. IEEE Ultrasonics Symposium, pp. 969-972 (2002) discloses a technique for producing a thin-film piezoelectric resonator in such a manner that a piezoelectric thin film sandwiched between upper and lower electrodes is formed on the front surface of a Si wafer and a cavity is formed in the rear surface of the Si wafer by silicon deep reactive ion etching (Si-Deep-RIE). Zinc oxide (ZnO), aluminum nitride (AlN) or the like is used in the piezoelectric thin film.
On the other hand, IEEEMTT-S Digest TH5D-4, pp. 2001-2004 discloses a thin-film piezoelectric resonator provided with an acoustic reflection layer instead of the cavity. A method for producing the thin-film piezoelectric resonator is as follows. First, a hollow is formed in a surface of a Si substrate. Films high in acoustic impedance and films low in acoustic impedance are laminated alternately on the surface of the Si substrate to thereby form an acoustic reflection layer. The acoustic reflection layer is then polished till the acoustic reflection layer is embedded in the hollow of the substrate so that the surface of the substrate is flattened. A lower electrode, a piezoelectric thin film and an upper electrode are laminated on the acoustic layer embedded in the hollow. Thus, a thin-film piezoelectric resonator can be finished.
In the thin-film piezoelectric resonator, the filter formed of the thin-film piezoelectric resonators and the voltage-controlled oscillator formed in the filter, there is however no consideration about the following point.
The thin-film piezoelectric resonator uses a structure in which a lower electrode, a piezoelectric material and an upper electrode are laminated successively on a cavity or acoustic reflection layer disposed in a substrate and in which an overlapping region among the cavity or acoustic reflection layer, the lower electrode, the piezoelectric material and the upper electrode serves as an excitation portion. The opening shape and opening size of the cavity or the planar contour shape and contour size of the acoustic reflection layer with respect to the planar shape and planar size of the excitation portion is affected by change in production process condition of the thin-film piezoelectric resonator. That is, if the production process condition changes, variation occurs in the frequency pass characteristic of filters formed of thin-film piezoelectric resonators produced before and after the change.
In the ladder-type filter 102 shown in FIG. 39, the number of ladder steps, the sequence of series and parallel connections, the capacitance value of each thin-film piezoelectric resonator 101, the length of connection wiring, etc. are important parameters on designing the filter function. It is necessary to adjust the input/output impedance of the filter to the requirement of the circuit to be designed. Generally, the input/output impedance of the filter is set to be 50Ω. The number of thin-film piezoelectric resonators 101 for forming one filter and respective capacitance values of the thin-film piezoelectric resonators 101 are decided in accordance with the requirement on designing the filter circuit. The capacitance value of each thin-film piezoelectric resonator 101 is proportional to the planar size (planar area) of the excitation portion. The ladder-type filter 102 is formed of a combination of thin-film piezoelectric resonators 101 having excitation portions of various planer sizes.
When thin-film piezoelectric resonators 101 having small capacitance values are combined on design of the ladder-type filter 102, it is not good to combine thin-film piezoelectric resonators 101 having excitation portions of small planer sizes. If the planar size of each excitation portion is small, the rate of the peripheral length to the planar area of the excitation portion becomes large. Increase in the rate of the peripheral length brings increase in the rate of release of vibration energy from the periphery of the excitation portion. In an excitation portion small in planar area relative to an excitation portion large in planar area, the resonance characteristic shows a tendency toward deterioration because of increase in energy loss. To avoid this tendency, excitation portions (thin-film piezoelectric resonators 101) having double capacitance values are connected in series to design the ladder-type filter 102 so that the planar area of the excitation portion of one thin-film piezoelectric resonator 101 is prevented from becoming too small.
Conversely, when a thin-film piezoelectric resonator 101 having a large capacitance value is required, the rate of release of vibration energy can be reduced but a risk of breaking caused by shortage of mechanical strength (film strength) supporting the excitation portion, deterioration of resonance characteristic caused by distortion, etc. occur in the thin-film piezoelectric resonator 101 having the excitation portion supported on the cavity. To avoid the risk of breaking and deterioration of vibration characteristic, excitation portions (thin-film piezoelectric resonators 101) having half capacitance values are connected in parallel to design the ladder-type filter 102 so that the planar area of the excitation portion of one thin-film piezoelectric resonator 101 is prevented from becoming too large. The planar area of the excitation portion however varies even in the case where the capacitance value of each excitation portion is adjusted on the basis of the requirement on design of the filtering characteristic or the requirement on the characteristic of each thin-film piezoelectric resonator 101.
From the viewpoint of the structure of each thin-film piezoelectric resonator 101, the opening size of the cavity or the contour size of the acoustic reflection layer needs to be larger by a length enough to attenuate vibration energy leaked from the periphery of the excitation portion than the planar size of the excitation portion. At the least, the cavity or acoustic reflection layer is provided to surround the excitation portion with a predetermined distance kept between the cavity or acoustic reflection layer and the excitation portion. In this case, in the ladder-type filter 102 formed of a plurality of thin-film piezoelectric resonators 101, cavities different in opening size or acoustic reflection layers different in contour size are produced in accordance with the thin-film piezoelectric resonators 101. As a result, the following problem occurs.
As shown in FIG. 41, in the process for producing the thin-film piezoelectric resonator 101 disclosed in JP 2000-69594 A, dishing 113 in which the hollow in the surface of each sacrificial layer 112 becomes large in the cavity 111 as the opening size becomes large occurs easily after the flattening and polishing process in which the sacrificial layers 112 are embedded in the cavities 111 different in opening size in the substrate 110. As shown in FIG. 42, erosion 114 in which the surface of the substrate 110 in the sacrificial layer 112 and its periphery in each cavity 111 is eroded as the opening size decreases occurs easily. Dishing 113 or erosion 114 deteriorates the flatness of the surface (surface of the substrate 110) of the sacrificial layer 112 to thereby deteriorate the crystallinity of the lower electrode and the piezoelectric material formed on the surface of the sacrificial layer 112.
Moreover, in the process for removing the sacrificial layer 112, the volume of the removed sacrificial layer 112 varies according to the opening size of the cavity 111. Accordingly, the etching time to remove the sacrificial layer 112 varies according to the cavity 111, so that it is difficult to set the end point of etching. Moreover, when etching advances based on a chemical reaction, the state of supply of etchant varies according to the arrangement position of the thin-film piezoelectric resonator 101 in the ladder-type filter 102, so that the etching speed of the sacrificial layer 112 varies according to the thin-film piezoelectric resonator 101. Specifically, the concentration of etchant varies locally in between the sacrificial layer 112 of the thin-film piezoelectric resonator 101 disposed in the center position and the sacrificial layer 112 of the thin-film piezoelectric resonator 101 disposed in the periphery of the center position, so that the etching speeds of the two are different from each other. It is difficult to set the end point of etching also because of such variation in etching speed. To remove all the sacrificial layers 112, an excessive time is taken for etching.
In the process for producing the thin-film piezoelectric resonator 101 disclosed in Proc. IEEE Ultrasonics Symposium, pp. 969-972 (2002), a micro-loading effect occurs easily at the time of etching when the cavity 111 is formed in the rear surface of the substrate 110 just below the excitation portion by the Si-Deep-RIE process after the excitation portion is formed by laminating the lower electrode, the piezoelectric material and the upper electrode on the substrate 110. The micro-loading effect is a phenomenon that the etching speed in the cavity 111 small in opening size becomes slower than the etching speed in the cavity 111 large in opening size. If the condition is set so that etching can be perfectly completed in cavity 111 small in opening size, the over-etching time in the cavity 111 large in opening size becomes so long that tunneling just above the stopper layer 115, that is, a notch 116 is generated easily as shown in FIG. 43.
Moreover, the etching speed varies according to the density of arrangement of the cavity 111 like wet etching. In the ladder-type filter 102, the etching speed in the sacrificial layer in the cavity 111 of one thin-film piezoelectric resonator 101 is different from the etching speed in the sacrificial layer in the cavity 111 of the other thin-film piezoelectric resonator 101 disposed around the first-mentioned thin-film piezoelectric resonator 101. Also in such a case, increase in over-etching time is brought according to the arrangement position of the cavity 111, so that the notch 116 is generated in the cavity 111 of the other thin-film piezoelectric resonator 101. Because the generation of the notch 116 changes the opening size of the cavity 111 to thereby change the parasitic capacitance value added to the excitation portion (piezoelectric capacitor), the frequency characteristic of the ladder-type filter 102 changes.
Moreover, in the ladder-type filter 102, because cavities 111 of thin-film piezoelectric resonators 101 are disposed adjacently, such notches 116 make the wall between adjacent cavities 111 so thin that the mechanical strength of the substrate 110 is weakened due to destruction of the wall as the case may be. Moreover, because the large notch 116 changes the opening end shape of the cavity 111 to a concavo-convex shape in which concentration of stress occurs easily, mechanical strength against breaking of the thin film of the excitation portion is weakened. Moreover, if the excitation portion abuts on the concavo-convex portion of the opening end caused by the notch 116, spurious response occurs in resonance characteristic so that the filtering characteristic of the ladder-type filter 102 deteriorates.
In the process of producing the thin-film piezoelectric resonator 101 disclosed in IEEE MTT-S Digest TH5D-4. pp. 2001-2004, the crystallinity of the lower electrode and the piezoelectric material formed on the surface of the acoustic reflection layer deteriorates because dishing 113 or erosion 114 occurs easily in accordance with the flattening and polishing process of the acoustic reflection layer embedded in the substrate in the same manner as in the process of producing the thin-film piezoelectric resonator 101 disclosed in JP 2000 69594 A.